All levels of packaging, (chip, electronic circuit assembly and systems) are becoming more miniaturized. A major technical challenge is thermal dissipation as power density increases. Typically, heat spreaders or heat sinks are bonded to microelectronic components and chips, individually. Occasionally, a common heat sink may be bonded to multiple chips on co-planer top surfaces. In many applications, space limitations and cost do not allow the use of heat sinks or co-planer electronic assemblies.
Flexible circuit boards offer advantages of light weight, thinness, three dimensional configuration, and flexibility. Space and height constraints for circuit packaging may dictate the use of flexible circuits over rigid laminates and very low profile chips and packages over high profile chip modules.
U.S. Pat. No. 3,781,596 discloses a single layer interconnection structure of metallic conductors on a polyimide film (e.g. KAPTON.TM. by E. I. by DuPont de Numers). U.S. Pat. No. 3,868,724 discloses metallic conductors sandwiched between polyimide film which project through the film.
Rigid-Flex circuit boards are described in U.S. Pat. No. 5,121,297 to Haas and U.S. Pat. No. 5,144,742 to Lucas. In those patents single layer flex circuit boards are integrally connected between rigid circuit boards. In both patents multiple rigid circuit board layers are laminated to a flexible circuit board substrate and components are attached only onto the rigid circuit board sections.
Connecting components directly to one side of a single-layer flexible circuit board is described by McBride, "Multi-function Plug for IC Package", IBM Technical Disclosure Bulletin Vol. 21, February 1979, pp. 3594-3595. I/O terminals on the bottom of a chip are soldered to pads on top of a thin polyimide flexible decal. Also, a depression in a heat sink cover is bonded onto the top of the chip. Connecting lower power chips to a bottom side of a multi-layer flexible circuit board and high power chips to the top side of the flexible circuit board in order to connect the high power chips to the module cap is suggested by McBride, "Multilayer Flexible Film Module", IBM Technical Disclosure Bulletin Vol. 26, May 1984, p. 6637. In that article I/O pins connect the flexible film to a metalized ceramic substrate, and smaller pins interconnect the layers of the film. Schrottke, "Removal of Heat from Direct Chip Attach Circuitry", IBM Technical Disclosure Bulletin Vol. 32, September 1989, pp. 346-348 describes a flexible circuit board with two rows of Direct Chip Attach (DCA) chips attached by controlled collapse chip connections (C4). The flexible circuit board is folded around a stiff heat spreader of copper-INVAR-copper (INVAR is a trademark of Creusot-Loire for a nickel-iron alloy) with the chips on the inside of the fold. The exposed surfaces of each row of chips are bonded with a thermally conductive adhesive to a respective major surface of the spreader.
U.S. Pat. No. 5,179,501 to Ocken, and U.S. Pat. No. 5,159,751 to Cottingham disclose bonding flexible circuit boards directly to one side of an aluminum plate. In Ocken, heat Is conducted through the circuit board to a first heat sink plate then to a second heat sink plate bonded to the first plate. "Concept for Forming Multilayer Structures for Electronic Packaging" (Curtis), IBM Technical Disclosure Bulletin Vol. 30, August 1987, pp. 1353-1356 and U.S. Pat. No. 4,811,165 to Currier, disclose folding a flexible circuit board around a heat sink plate and bonding the board to both sides of the plate. In both citations, before bonding the board to the plate, components are connected to the side of the board which is not connected to the plate. "Integral Heatsink Printed Circuit Board" (Askalv), IBM Technical Disclosure Bulletin Vol. 25, December 1982 p. 3606, discloses a flexible circuit board adhesively bonded to a heat sink of steel or aluminum.
U.S. Pat. No. 5,168,430, discloses a portion of a flexible circuit board applied to a heat sink plate and provided with a cutout in which a hybrid circuit structure is cemented to the plate. The hybrid circuit structure is wirebonded to the flexible circuit board.
U.S. Pat. No. 4,834,660 to Cottiat, FIG. 9, shows a circuit board interconnection system in which layers of a flexible circuit board are removed in part of a bend to expose a selected wiring layer for connection. The flexible circuit board is of copper clad polyimide.
All the above citations are hereby incorporated by reference.